The effect of chilling on membrane lipid phase transition in human oocytes and zygotes

Background: Chilling injury occurs when the cell membrane undergoes a transition from the liquid state to the gel state. Human oocytes and single-cell zygotes are of similar shape and size but the post-thawing survival rate of oocytes is poorer. We set out to investigate the possible difference in membrane lipid phase transition (LPT) temperature between the two cell types. Methods: The LPT temperature was measured with a Fourier Transform Infrared analyser, which detects the change in the vibration frequency of the CH₂ bond stretches of the membrane lipid molecules during temperature change. The LPT temperatures of unfertilized human oocytes, in vitro-matured oocytes, and immature germinal vesicle (GV) stage oocytes were compared with that of abnormally fertilized human zygotes. Results: The LPT temperatures of zygotes and of mature and immature GV oocytes differ significantly from each other (10.0 ± 1.2, 16.9 ± 0.9 and 24.4 ± 1.6°C respectively; P < 0.05). Conclusions: Zygotes show a higher resistance to chilling injury compared to oocytes at different developmental stages; this might explain the relatively poor survival rates of cryopreserved human oocytes and indicates the necessity to adjust the cryopreservation protocols in order to minimize cryoinjury.